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Aerodynamics
Introduction The exterior of the car is one of the most important aspects of solar car design. Why? *The exterior of a solar car inherently has conflicting design goals. The need to provide a suitably large and flat mounting surface for a solar array is in direct opposition with the requirement for low drag. Finding an acceptable balance between these two goals takes a lot of time, and involves a lot of computation. *The power required to push the car through the air goes up with the cube of speed. P_{aero} = \frac{1}{2} \rho V^3 C_D A . A car with poor aerodynamics will require an extremely large amount of power to drive at highway speeds. *The shell of the car is likely the single heaviest component of the car (driver excluded), and as such, is one of the places that designers should concentrate on cutting weight. You can only cut ounces out of suspension components, but you can cut tens of pounds out of the composite structures of the shell. *The manufacturing process for the exterior of the car is likely one of the longest ones on the car. For example, the University of Minnesota cuts the molds for their shell 9 months before the completion of the car. Array Considerations Wheel Fairings A spinning wheel in an airstream creates an incredibly large amount of drag; therefor it is crucial to mask the wheels of a solar car from the airflow if you wish to build an aerodynamic car. Fairing in wheels that don't steer is a fairly trivial exercise, so we're not going to discuss it much here. The real trick is covering the front wheel(s) of the car that steer. Wheel fairings fall into two broad categories: Static and Dynamic. Static Fairings Static fairing do not move at all. A static fairing that encompasses the whole wheel will have to be wide enough to allow the wheels to steer from lock to lock. Alternatively, a skinny leading edge and trailing edge can be grafted to the body of the car ahead and behind the wheel, with the sides of the wheel open to the airstream. In both of these cases, and "hourglass cut" (The shape of a wheel turning from left steering lock to right lock) in the bottom of the car is open of the airflow. There are ways to mask this cutout, but if you're going to go through the trouble of designing a mechanism/flexible surface to to seal it, you might as well have just gone on to build a full dynamic fairing. Dynamic Fairings Dynamic fairings move or open in some manner to accommodate the wheels when the car is turning, while maintaining a much skinner profile while the car is driving straight. Canopy The driver needs some way to see out of the car. Some teams design a "bubble" around the driver's head, while other teams blend the canopy into the main body shell. This is an aerodynamics versus solar array area/shape/shading issue that should be analyzed and optimized. In addition, many (most?) teams use a canopy hatch as the primary method of driver egress; this should be taken into account when thinking about the array around the canopy. Material choice for windshield: Lexan vs. Acrylic. Analysis CFD Like all Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD) results are only as good as the input data. Garbage in, garbage out. If the operator is not skilled, the chances are fairly high that pretty pictures and meaningless data will result. Even a skilled engineer may have trouble getting accurate drag results while simulating solar cars, as most of the drag for vehicle like this is the result of skin friction, which is hard to model. On the other hand, it shouldn't be hard to get reasonably accurate lift/downforce numbers, so CFD is still useful for tuning the desired amount of camber in a design. On the other hand, if the same mesh settings are used for multiple CAD models, CFD is very useful for comparing drag numbers of difference designs to each other, even if the absolute values are incorrect. It is also useful to be able to quantitatively look at the flow for odd vortexes or separation; this may point to areas that can be revised and improved in the design. Wind Tunnels Wind tunnels can be used to analyze scale models (or full size models, if you have access to a big enough tunnel). A critical issue is time and raw materials. If a scale model is used, recall what you've learned about dimensional analysis. Road Testing When all is said and done, the only way to get accurate numbers for the power consumption of the car is to take it out and drive it.